Shortly after his nomination was made, Republican leaders in the state Senate announced their support of their former Democratic colleague. McDonald served in the General Assembly for eight years.

‘‘While Andrew and I have had considerable political differences over the years, I have always respect­ed his commitment to public service and the law,’’ said Senate minority leader John McKinney, Republican of Fairfield. ‘‘He is qualified, and I am confident he will uphold the state constitution and carry out his responsibilities as a Supreme Court justice with the highest degree of impartiality and integ­rity.’’

As cochairman of the General Assembly’s Judiciary Committee, McDonald was a chief proponent of same-sex marriage and an earlier law legalizing civil unions for gay and lesbian couples.

For most of his legal career, McDonald worked as a litigation partner for Pullman & Comley LLC, where he chaired the law firm’s appellate practice. He also served as director of legal affairs and corporation counsel for the city of Stamford, from 1999 to 2002. That was during Malloy’s tenure as mayor of Stamford.

On Thursday, McDonald ­recalled being humbled by the experience of first arguing ­before the state Supreme Court.

‘‘The notion that one day I might sit as a member of that court is something I couldn’t have imagined,’’ he said, thanking Malloy for the nomination.

Malloy officiated at McDonald’s wedding three years ago, after the state’s highest court ­legalized gay marriage in ­Connecticut. McDonald is married to Charles Gray.

McDonald is expected to replace Justice Lubbie Harper Jr., who has reached the mandatory retirement age of 70. Malloy said he will name a replacement for Justice C. Ian McLachlan, who has also reached the retirement age, in the coming weeks.